The occupant sensor device that uses capacitance may include, for example, a capacitance type sensor and an occupant detection electronic control unit (hereinafter “ECU”). The capacitance type sensor is a device that outputs, either as an electric current or a voltage, a change in a weak electric field that is generated between a main electrode disposed in a seat and a vehicle body. The occupant detection ECU distinguishes whether an occupant is seated and what type of occupant is seated. In particular, whether an adult, a child on a child seat (i.e., a child restraint system: CRS) or other objects is seated is determined based on the change of the capacitance. Such an occupant sensor device that uses capacitance is described in Japanese Patent Laid-Open No. H11-271463.
In a course of such determination, many irregular cases are observed. For instance, when an adult with small stature sits only on a front portion of the seat (i.e., having “a front-side sitting” posture), or when an adult sits on one side (i.e., on the right/left side) of the seat (i.e., having “a sideway sitting” posture), the output value of the capacitance from the capacitance type sensor decreases.
Therefore, a difference between a capacitance output of the front-side sitting or the sideway sitting of a small stature adult and a capacitance output of a child (i.e., an infant) sitting on CRS may be very small. As a result, the distinction between a smaller stature adult and a CRS-cradled infant may be very difficult to make.